In many businesses, and particularly in the business of vehicle body and frame repair, it is necessary to bend, pull and shape metal, either to restore it to its original shape or to put it into another desired shape. Although a number of manufacturers have made various devices for use in body and frame repair, such devices essentially employ the same technique. That is, they each include hydraulic ram, an upright post, and a working element such as a chain which is connected to the body or frame at one end and to the post at the other end. Pulling force is produced by the hydraulic ram and is transmitted to the chain and thence to the body or frame to be straightened.
Although a great amount of force can be produced by such previously known devices, and the force can be transmitted to the body or frame, such devices have certain inherent limitations and disadvantages. For example, the force transmitted to the body or frame part to be shaped is not constant. Rather, each time the hydraulic ram is operated the force transmitted to the body or frame varies from a great amount to a very small amount and then to a great amount again. This results from the nature of the operation of the hydraulic ram and also from the fact that the post and chain are made of metal and are extremely rigid under tension. Another problem inherent in the use of such devices is that the metal being straightened or shaped tends to harden when it is bent or shaped more than once, thereby weakening the metal. For example, when the metal is pulled a certain distance, then relaxed, and then pulled again, the metal tends to harden undesirably.
Another related problem associated with the use of such device is that the metal may be inadvertently pulled too far, thus requiring additional time and labor to push the metal back to the proper shape. Of course, this increases the tendency for the metal to harden undesirably and it also increases the possibility that the metal will be wrinkled or slightly mis-shaped.
A further problem associated with the use of such existing devices occurs when the chain, or the attachment of the chain to the body or frame, fails. Because of the tremendous pulling forces imposed on the chain, when it fails (i.e., breaks or detaches from the body or frame) the chain can become a lethal weapon when it snaps back toward the post away from the body or frame.